EnhancedNullability annotation in IR

Fixes KT-40115 & KT-40117.

Move FlexibleNullability annotation to 'kotlin.internal.ir'.

compiler/testData/codegen/bytecodeListing/nullabilityAnnotations/lateInitNotNull.kt

@@ -0,0 +1,5 @@
+// Test to ensure that we mark the backing field of a lateinit property
+// as NotNull, even though the field is nullable in the JVM IR backend.
+class A {
+    lateinit var x: A
+}